Physical properties of group 16, 17 and 18 elements

a. Group 16 elements (Oxygen family or chalcogens) : Oxygen is a gas while the other elements are solids at room temperature. Oxygen and sulfur are nonmetals, selenium and tellurium are metalloids, while polonium is a metal. Polonium is radioactive, with a half life of 13.8 days. Melting and boiling points increase with increasing atomic number. All the elements of group 16 exhibit allotropy.

b. Group 17 elements (Halogen family) : Fluorine and chlorine are gases, bromine is a liquid and iodine is a solid at room temperature. $\mathrm{F_2}$ is yellow, $\mathrm{Cl_2}$ greenish yellow, $\mathrm{Br_2}$ red and $\mathrm{I_2}$ is violet in colour. Fluorine and chlorine react with water. Bromine and iodine are only sparingly soluble in water, and are soluble in various organic solvents such as chloroform, carbon disulfide, carbon tetrachloride and hydrocarbons, in which they give coloured solutions. Bond dissociation enthalpies of halogen molecules follow the order:

$\mathrm{Cl\text{-}Cl > Br\text{-}Br > F\text{-}F > I\text{-}I}$ …